I've removed all the spoilers and only left in teasers, enjoy...
The film is based on a book by deservedly famous mystery/thriller author Dennis Lehane, who has already had two of his books turned into critically acclaimed films, "Mystic River" and "Gone, Baby, Gone." Lehane's "Shutter Island" was optioned in the early years of the decade and languished in various stages of development for some time. Eventually Scorsese became attached when the project was taken over by Paramount Pictures, with a screenplay by Laeta Kalogridis. That's a bit of a concern. Kalogridis' previous features are "Pathfinder" and "Alexander," neither of which were really good at all. She was also a writer/producer of the miserably awful and blessedly short-lived series "Bionic Woman," and "Birds of Prey" before that, but past performance is often not a predictor.
Lehane has a near-unimpeachable reputation among mystery fans, although opinions vary as far as this book is concerned. It has the same moodiness, angst and mysterious atmosphere as his other books, water-soaked and suffused with ocean-scented Boston darkness, but its ending gives a lot of people some trouble.
Then there's the ending. I've found some discussion of the ending among fans of the book; apparently it was ambiguous and difficult to understand, in the way of "Mulholland Drive" or "Donnie Darko," (not a bad comparison).
Mark Ruffalo has high praise for the film; he's quoted as saying that he thinks it'll be a major stylistic change for Scorsese and will be wildly successful.
Scorsese hasn't really directed in the suspense/thriller genre since "Cape Fear," which I certainly found terrifying. He's a master of suspenseful drama with a lot of tension even in his non-thriller films, and this material seems very well suited to Scorsese's style.